Harmonic drive reduction gears and oscillating bevel reduction gears are well known as reduction gears capable of realizing a large reduction ratio.
The former, i.e., the harmonic drive reduction gear has a circular spline and a Flexspline which is two less in teeth number than the circular spline. The gear is a mechanism which continuously bends the Flexspline with a wave generator and thereby the Flexspline and the circular spline are engaged at two places (see Japanese Patent Application Publication Number 7 (1995)-103291).
The harmonic drive reduction gear has advantages of prevention of occurrence of backlash and smooth rotation by the engagement at the two places. However, the difference in teeth number needs to be set to an even number equal to or more than two from construction condition. Therefore, there is a problem that a reduction ratio is 2/N (N: teeth number) at most. In the harmonic drive reduction gear, there are issues that a large motor is required and thereby the whole mechanism grows in size and also causes weight increase, because a starting torque of gears also becomes large.
The latter, namely oscillating bevel reduction gear has a fixed bevel gear and a movable bevel gear between which difference in teeth number is one. The gear is a mechanism configured so that the movable bevel gear is pressed on the fixed bevel gear with an inclined disk united with an input axis and they perform precession movement while engaging with each other at one place (see Japanese Patent Application Publication Number 7 (1995)-248047).
In the oscillating bevel reduction gear, as the difference in teeth number is one, a reduction ratio can be set to 1/N (N: teeth number), it has an advantage capable of realizing a high reduction ratio. However, as the bevel gears are placed in contact at one place, there is a problem that rotation smoothness is reduced and backlash tends to take place. There are issues that the size in the axial direction is increased and thereby the whole mechanism grows in size and also causes weight increase, because the bevel gears are also combined with each other.
That is, the known harmonic drive or oscillating bevel reduction gear has difficulty in simultaneous pursuit of realizing a high reduction ratio, preventing backlash by rotation smoothness, compacting its entire mechanism and saving weight. Therefore, they are not suitable to be embedded in a knuckle of a robot hand or the like.